This invention relates to electro-hydraulic steering systems for wide track vehicles such as straddle cranes which have two or more steerable wheels connected to the vehicle frame. Although it is not uncommon for such vehicles to use hydraulic steering and propulsion or motive force controls, prior art systems often do not provide the necessary means for maintaining optimum turning radii for the steerable wheels when the vehicle operator is attempting to maneuver the vehicle along a curved path. Such prior art systems are not easily adaptable for use with two wheel steering, four wheel crab steering, and four or more wheel coordinated steering, particularly when conventional tie rods or four bar linkage systems are used.
In large, wide track vehicles, particularly those of the straddle crane type such as used to carry heavy or awkwardly sized loads, the visual attention of the operator is often fully occupied during vehicle maneuvers with the need to observe clearances to prevent collisions with obstacles close to the desired path of vehicle travel. Accordingly, it is essential that the steering controls provided to the vehicle operator be simple and be adaptable to achieve optimal vehicle control while requiring minimum attention or inputs from the operator. In particular, it is desirable for vehicles of the type described to permit the operator to control a single wheel and to provide a mechanism which automatically steers the remaining wheels accordingly.
The prior art has produced several systems which provide a control mechanism for accurate tracking by the wheels of a multi-wheel vehicle under the control of a single operator. For example, U.S. Pat. No. 3,387,684 discloses an electronic steering apparatus which generates electrical signals corresponding to the selected direction of travel. A wheel control system responds to the electrical signals to attempt to maintain each wheel normal to the turning radius. A mechanical linkage controlled by an electro-hydraulic cylinder thus operates to control tracking of a wheel. Similarly, U.S. Pat. No. 3,532,178 discloses a system for steering independently steerable wheels of a vehicle in response to the rotation of the steering wheel or other control member to provide the desired steering geometry. In addition, U.S. Pat. No. 3,572,458 discloses a system for individually steering all wheels of a heavy vehicle by use of a cam-controlled servo transmitter.
Nevertheless, it has remained a problem in this art to provide a steering system which electro-hydraulically steers a plurality of automatically steered wheels while eliminating complex mechanical linkages for the steerable wheels, such as tie rods, four bar linkages and the like. Moreover, it is desirable to provide a steering system which is electro-hydraulically responsive to cause the steerable wheels to assume an optimum turning position.
Accordingly, it is an object of the invention to provide a hydraulic steering system for a wide track vehicle wherein automatic control means are provided to cause the slave steerable wheels to turn appropriately relative to one another and relative to the vehicle frame during a turning maneuver in response to the position of a master steerable wheel so that optimum wheel angularity will be maintained for all turning wheels to minimize tirescuffing, optimize wheel traction, and reduce frame stress.
It is another object of the invention to provide automatic control means for incremental control of the relative position of the steerable wheels of the vehicles during a turning maneuver to achive an optimum turning condition for all wheels as a function of the position of a master steerable wheel and the position of a slave wheel.
It is a still further object of this invention to provide for the foregoing with a relatively simple and cost effective system which requires a minimal amount of manual operator attention for effective operation and avoids tie rods and mechanical linkage components.
These and other objects, aims and objectives of this invention will become apparent from a review of the following written description of the invention taken in conjunction with the accompanying drawings.